Several important chemical processes use a high temperature furnace to drive catalytic reactions. Examples include steam reforming of light hydrocarbons for the manufacture of synthesis gas for ammonia, methanol and hydrogen plants, and steam pyrolysis of saturated hydrocarbons to produce olefins. Such furnaces generally include a refractory-lined radiant heating combustion chamber with an arrangement of burners and process stream heat transfer tubes. Combustion product gases are directed from the combustion chamber through a flue gas convection section where the bulk of the waste heat remaining in such gases is extracted by forced convection against other process and utility streams prior to discharge.
Fundamental elements of furnace design are well known and have changed very little. Efforts to improve furnace efficiency have concentrated on the heat transfer equipment, burner design and arrangement and the combustion process itself. For example, combustion improvements have included adjusting reactant ratios, catalyst and furnace firing of the radiant section.
U.S. Pat. Nos. 4,999,089 to Nakase et al.; 4,706,612 to Moreno et al.; and 4,784,069 to Stark describe typical furnace arrangements of radiant and convection heating sections known in the art. Moreno utilizes gas turbine exhaust to preheat and supplement furnace combustion air. Stark also uses turbine exhaust as a low oxygen source of combustion air for operating a furnace in a fuel-rich method.
The use of gas turbine exhaust as part or all of the combustion air in a furnace results directly in reducing overall fuel requirements. However, the combustion product gases from a furnace operated in this manner are greater in volume, but lower in temperature than flue gas from a conventionally operated furnace. The large volume, low temperature flue gas complicates waste heat recovery because the temperature approaches between the flue gas and the process and/or utility stream being heated are too close to allow efficient or economical heat recovery, if at all.
U.S. Pat. No. 4,665,865 to Zubrod describes a steam generator in which a first flue has an upper end with a cross flue leading to a second flue. The cross flue is formed by an intermediate ceiling in the first flue and two vertical walls made of pipe.
U.S. Pat. No. 3,097,631 to Martin describes a means for controlling the flow of heating gas through the flue of a steam boiler or water heater. Combustion product gases conducted along a switchback main duct path can enter a damper controlled bypass duct having a series of dampers allowing part of the gases to selectively bypass part of the main duct path.
U.S. Pat. No. 2,721,735 to Permann describes a furnace wherein the heating tubes are arranged within a heating zone so as to cause combustion gases to flow longitudinally to the tubes and part of the gases are recirculated to the burners.
Other U.S. Pat. Nos. of interest are 3,426,733 and 3,424,695 to Wiesenthal; and 3,094,391 to Mader.